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Comment from S. Pollock (CU Boulder, visiting OSU and teaching Paradigm “Vector Fields), Nov 2009:

This activity can take a lot more time than one would expect, depending on how much “setup” is provided ahead of time.

I decided to emulate the original implementers and NOT do much setup, and discovered that these students did not know Gauss law at *all* (turns out it was not covered for many of them in their intro class, at all!) so asking them to discover the method here (using Gauss to find E in symmetric cases) for themselves was asking too much. I pushed on explanations, so e.g. almost all groups cheerfully concluded E=0 inside “the cavity” (spherical or cylindrical), because q_enclosed=0, but when pressed, could not articulate any good reasoning (and e.g. when I pointed out that q_enclosed for the cubical surface we had just looked at in a MAPLE activity was zero when the charge was *outside*, but E was certainly not zero THERE, they realized that much more was needed, still without yet knowing how to proceed.

Seems to me that either one would need a LOT of time and TA support to guide students through this (I'm thinking 2 full hours), or else you might need/want to do a sample problem (e.g. sheet of charge, cubic surface?) ahead of time, which would take away some of the discovery elements of this activity, but would allow the students to generate new surfaces, think about issues of symmetry and area and integration that we want, without taking quite so much time. Thoughts from others?

-S